Safety device for controllably driving the cassette holder of a video cassette recorder subjectable to overload

ABSTRACT

Motor control apparatus comprised of a drive circuit for supplying drive current to a motor, an overload detector coupled to the motor for detecting the magnitude of the motor current to produce an output signal if that current exceeds a predetermined threshold level caused by motor overload, and a control circuit coupled to the drive circuit and responsive to the output signal for controlling the drive current supplied to the motor, thereby controlling motor operation.

This application is a division of application Ser. No. 07/398,967, filedAug. 28, 1989.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to motor control apparatus and, moreparticularly, to a circuit which is adapted to sense a motor overloadcondition whereupon motor operation is controlled to alleviate thatcondition, such as by reversing the motor, reducing the drive currentsupplied to the motor or interrupting the drive current so as to stopthe motor.

2. Description of the Prior Art

Motor control circuitry has long been used in driving, regulating andcontrolling the operation of an electric motor. Often, such motorcontrol circuitry is intended to provide protection against motor damageand, for those applications in which the motor is used withuser-operated devices, to prevent personal injury to the user.

One example of a controllable motor adapted for use in a humanenvironment is found in recording/reproducing apparatus, such as a videotape recorder (VTR). Many commercially available VTR's employ a cassetteholder to receive a tape cassette which then is loaded into the VTR.Typically, an electric motor is mechanically coupled to the cassetteholder and is driven to move that cassette holder between a load/unloadposition whereat the tape cassette may be loaded onto or unloaded fromthe holder, and a record/playback position whereat the tape cassette isbrought into operable relationship with electromechanicalrecord/reproducing apparatus. Usually, the simple actuation of a STARTswitch energizes the motor to drive the cassette holder, either from itsload/unload position to its record/playback position or from itsrecord/playback position to its load/unload position. It is recognized,however, that uncontrolled motor operation may lead to personal injuryof the user. For example, the user may accidentally locate his finger inthe path of the cassette holder as it moves from its load/unloadposition to its record/playback position, resulting in possibly severedamage or painful injury.

To minimize this risk of personal injury, various safeguards have beenproposed to prevent the user from inadvertently placing his finger inthe path of motor-driven apparatus. Unfortunately, even the mostdetailed precautions often are defeated or, at the least, result incomplicated and often difficult manipulation by the user in loading orunloading a tape cassette from its holder. Accordingly, as analternative, protection arrangements have been proposed which stop acassette loading operation should the user's finger become entangled inthe equipment. While this may prevent further injury, such arrestingapparatus often relies upon mechanical elements which, while stoppingthe movement of the drive motor nevertheless does nothing about thecontinued supply of drive current thereto. As a result, the motor may besubjected to a current overload condition in which excess motor currentflows therethrough. Consequently, the motor may be damaged and may burnout.

Another difficulty associated with mechanical protection devices isattributed to the fact that such devices often are quite large and bulkyand are not easily incorporated into portable VTR's, such as videocameras. In particular, 8mm video cameras which record on and play backfrom an 8mm cassette are quite small and cannot easily accommodate theaforementioned protection devices.

In recognition of the danger of motor overload, some relatively simpletechniques have been proposed to prevent the motor from burning out dueto excess motor current drawn thereby. For example, the motor may bedesigned to exhibit limited torque which also limits the current; or themotor drive circuit may impose an inherent limitation on the maximumcurrent that may be supplied thereby. Such proposals limit the powerthat can be exerted by the motor and often require a motor and motordrive circuit that are relatively expensive to design and manufacture.

One simple alternative to special motor design is the use of a fuseconnected in series between the power supply from which the motor isdriven and the motor drive circuit, or even the motor itself. While afuse may prevent the motor from drawing excess current and, thus, avoidburn-out, a "blown" fuse must be replaced, and this may result infrequent servicing of the motor-driven equipment. In the environment ofconsumer equipment, it may be difficult for a user to carry out even therelatively simple task of replacing a fuse, thus making it moredifficult for the user himself to maintain and service his equipment.

An alternative to the use of a simple fuse relies upon a variable powersupply which is controlled to reduce the current supplied thereby to themotor when an overload condition is sensed. For example, when overloadis sensed, the power supply voltage may be reduced to prevent motorburn-out. However, if a common power supply is used to provide energyboth for the motor and for other circuits used with the motor-drivenequipment, a change in the power supply voltage which is intended toprevent motor burn-out may induce malfunctions or faults in othercircuitry that is supplied with that same power supply voltage. To avoidthis possibility, a separate power supply is needed for the exclusiveuse of the motor and motor drive circuitry. This, however, is expensive,adds to the complexity of the overall equipment, and increases the sizeand weight of the motor-driven apparatus.

One proposed protection circuit which is intended to prevent motorburn-out yet does not require a separate power supply and does not varythe power supply voltage that is used by other circuitry connects acurrent limiting circuit in series between the motor drive circuit andthe power source from which motor current is derived. The currentlimiting circuit includes active devices, such as FET's, bipolartransistors, or the like, and functions to limit the current flowingfrom the power supply to the motor. The current limiting circuit issensitive to current flowing therethrough to the motor such that whenthis motor current increases in magnitude, the impedance of the currentlimiting circuit likewise increases. Hence, the current permitted toflow through this circuit to the motor is limited so as not to exceedsome predetermined level.

Unfortunately, the use of a current limiting circuit of theaforementioned type increases the power dissipation of the overallcircuitry due to the fact that the impedance connected in series withthe power supply increases. Furthermore, the use of transistors andother active elements in the current limiting circuit inherentlydissipates power even when such devices are quiescent, as when the motorcurrent is well below the aforementioned predetermined level.Consequently, if the motor and motor protection circuitry are used inportable equipment, the battery pack power source normally used withthat equipment will exhibit a shorter effective life because of theincreased power dissipation in the current limiting circuit. Moreover,such current limiting circuitry is rather bulky and, thus, is unsuitablefor portable, compact equipment such as portable video cameras.

OBJECTS AND SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provideprotection circuitry for use with motor-driven apparatus to preventinjury to a user, damage to the motor and damage to the equipment withwhich that motor is used.

Another object of this invention is to provide motor control apparatusfor controlling the operation of a motor in the event of an overloadcondition, yet avoids the aforenoted disadvantages, drawbacks anddefects of prior art proposals.

A still further object of this invention is to provide motor controlapparatus which functions to limit the current supplied to a motor so asto prevent motor overload, yet is compact, does not dissipatesignificant amounts of power and is well-suited for use in portablemotor-driven equipment, such as a video recorder.

An additional object of this invention is to provide motor-driven tapecassette loading apparatus in which a motor controller detects motorload conditions so as to control that motor, such as by stopping orreversing it.

Various other objects, advantages and features of the present inventionwill become readily apparent from the ensuing detailed description, andthe novel features will be particularly pointed out in the appendedclaims.

In accordance with this invention, motor control apparatus is comprisedof a drive circuit for supplying drive current to a motor, an overloaddetector for detecting the magnitude of motor current so as to producean output signal if that current exceeds a predetermined thresholdlevel, as may be caused by motor overload, and a control circuitresponsive to the aforementioned output signal for controlling the drivecurrent supplied by the drive circuit to the motor.

As one aspect of this invention, the overload detector includes anenable circuit selectively operable during certain modes when motoroverload is to be detected.

As a feature of this invention, the drive circuit includes a powersource, variable impedance switches selectively energized for couplingthe motor to that power source, and a bias circuit for varying theimpedance of those switches to limit the current supplied through suchswitches to the motor. As an aspect of this feature, the aforementionedoutput signal produced by the overload detector is coupled to the biascircuit for varying the impedance of the variable impedance switches.

As another aspect of this feature, the variable impedance switches areconnected as plural transistors (e.g. FET's or bipolar transistors) inH-configuration to couple the motor to the power source. The transistorsare selectively energized to supply current to the motor in forward orreverse directions. The transistors may be further operated to brake themotor or to interrupt motor current such that the motor slows and stops.

In accordance with another feature of this invention, the controlcircuit includes a timer for sensing the presence of the aforementionedoutput signal for a predetermined time duration before controlling themotor drive current.

As yet a further feature of this invention, the overload detectorincludes a current level generator for generating a current level signalrepresentative of the motor current, a reference generator for providinga reference signal representative of a reference motor current, and acomparator for comparing the current level signal to the referencesignal to produce the aforementioned output signal if the current levelsignal exceeds the reference signal. Preferably, the current levelgenerator comprises a resistor connected in series with the motor toproduce a voltage thereacross in response to the motor current.

In one advantageous application of the present invention, the controlledmotor is used in a tape cassette loader by which a tape cassette isloaded and unloaded in a record/playback device, such as a VTR, videocamera, or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the present invention solely thereto, will best beunderstood in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of motor control circuitry which incorporatesthe present invention;

FIG. 2 is a partial block, partial schematic diagram of one embodimentof the present invention;

FIG. 3 is a partial block, partial schematic diagram of anotherembodiment of a portion of the present invention;

FIG. 4A is a perspective view of a portable video camera having amotor-driven cassette loader in which the present invention finds readyapplication;

FIG. 4B is a perspective view of the video camera illustrating onepossible cause of motor overload; and

FIG. 5 is a partial block, partial schematic diagram of a prior arttechnique for limiting the current supplied to a motor so as to preventmotor overload.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Before discussing the present invention in detail, reference first ismade to FIG. 5 which illustrates a prior art technique for preventingmotor overload by limiting the current supplied to a motor. In thisprior art circuit, motor current is supplied from a power supply 115 toa motor 120 by a motor drive circuit 110. A current limiting circuit 100is interposed in the series circuit comprised of the power supply, themotor drive circuit and the motor. As shown, motor drive circuit 110 isformed as a so-called H-bridge of transistor pairs with motor 120connected between transistors 200 and 230 of one pair and also betweentransistors 210 and 220 of another pair. A control logic circuit 125responds to command signals from a controller (not shown) to selectivelyenergize pairs of transistors. For example, when control logic circuit125 energizes transistor pair 200, 230, current flows from power supply115 through current limiting circuit 100, through transistor 200,through motor 120 and through transistor 230 to ground. As a result,motor 120 is driven in a first direction, such as the forward direction.

If the command signals command reverse motor drive, control logiccircuit 125 energizes transistor pair 210, 220, whereupon current flowsfrom power supply 115, through current limiting circuit 100, throughtransistor 210, through motor 120 and through transistor 220 to ground.It is seen that current now flows through motor 120 in the oppositedirection, thus reversing the direction of motor movement. The commandsignals may command a dynamic braking operation, whereupon eithertransistors 200 and 210 are turned on or transistors 220 and 230 areturned on. Whichever pair of transistors is energized, the terminals ofmotor 120 are supplied with the same voltage, thus providing ashort-circuit to dynamically brake the motor.

Finally, the command signals may command a stop operation, whereby motor120 coasts to a stop. In response to this command, control logic circuit125 de-energizes all of the transistors, thus interrupting current flowto motor 120.

Current limiting circuit 100 includes active devices, namely,transistors, field effect transistors (FET's) and the like. As currentflows through these active devices, the impedance exhibited therebyincreases. This has a self-limiting effect on the current supplied frompower supply 115 to motor 120 because as the current level tends toincrease, as by reason of a motor overload condition, the impedance ofthe current limiting circuit likewise increases which, in turn, opposesthe increase in current levels. For example, if the movement of themotor is constrained, as by reason of an obstruction in the path ofmovement of a device driven by that motor, the current supplied to motor120 tends to increase. But for current limiting circuit 100, thisincrease in current could reach an overload condition, whereupon themotor may burn out. But, by reason of the self-limiting effect of thecurrent limiting circuit, such high current levels are avoided.

Unfortunately, current limiting circuit 100 is a source of powerdissipation even when acceptable current levels are drawn by motor 120.If power supply 115 is comprised of a battery, such as if the apparatuswith which the illustrated circuitry is used is portable equipment, suchpower dissipation drastically reduces the effective life of thatbattery. Furthermore, current limiting circuit 100 normally is ratherbulky and may not be suitable for compact, portable equipment, such asportable video cameras having a VTR driven by motor 120.

The disadvantages associated with the prior art circuitry shown in FIG.5 are avoided by the present invention which is illustrated by the blockdiagram of FIG. 1. In the following description, like reference numeralsare used in different drawing figures to identify the same components.

The motor control circuitry 1 shown in FIG. 1 is comprised of a motordrive circuit 3 which drives motor 2 with drive current, the magnitudeof this current being sensed by a current detector 4. The overalloperation of the motor drive circuit and current detector is controlledby a controller 5. As will be described in greater detail below, motordrive circuit 3 includes drive transistors, FET's, or the like which areselectively energized to supply current to motor 2 connected thereto.The conductivity or impedance of these drive elements is controlled by abias circuit, thereby determining the magnitude of the current whichflows through motor 2. In one embodiment, these drive elements areconnected in pairs to the motor in an H-configuration; and respectivepairs of these drive elements are selectively energized to determine thedirection of current flowing through the motor. Controller 5 is coupledto motor drive circuit 3 and is adapted to select those transistor pairsto be energized and, thus, the controller determines the operatingdirection of the motor.

Current detector 4 is adapted to detect the magnitude of the currentflowing through motor 2 and to compare this detected current level to areference level representing a maximum, permissible motor current. It isappreciated that the motor may be considered to be overloaded when itscurrent exceeds this amount. Of course, excess currents may be drawn bymotor 2 even when the motor is not overloaded, such as during a start-upoperation, and current detector 4 is designed to be immune from suchnon-overloaded, high current conditions.

An overload output signal is supplied to motor drive circuit 3 fromcurrent detector 4 when motor 2 is overloaded. This output signal servesto increase the impedance of the motor drive elements and thereby reducethe current supplied to the motor. In one embodiment, the output signalfrom current detector 4 is supplied to a bias circuit included in themotor drive circuit; and the bias circuit functions to adjust the biasvoltage supplied to the drive elements so as to increase theirimpedances.

The overload output signal produced by current detector 4 also issupplied to controller 5 which responds thereto to generate controlsignals for motor drive circuit 3, thereby providing further controlover the motor current supplied to motor 2. Preferably, the controllerdetermines which of the drive elements included in the motor drivecircuit are to be energized, and this, in turn, defines the direction ofcurrent flowing through motor 2, whether the motor is to be braked, andwhether the motor is to be stopped. For example, controller 5 mayrespond to the overload output signal produced by current detector 4 tocommand a dynamic braking operation of motor 2. Alternatively, thecontroller may command a stop operation, as by interrupting the currentsupplied to the motor. As a still further alternative, controller 5 maychange the pair of drive elements which have been energized in the motordrive circuit, thereby supplying current in the reverse direction toreverse the operation of the motor. Depending upon the environment inwhich the motor is used, any of these alternatives may be adopted torelieve the motor overload. Of course, as yet another alternative, itmay be sufficient merely for the overload output signal produced bycurrent detector 4 to be used by motor drive circuit 3 to increase theimpedance of the drive elements and, thus, reduce the magnitude of thecurrent supplied to the motor.

Controller 5 also is adapted to supply an enable signal to currentdetector 4 to enable the current detector to sense a motor overloadcondition during certain operating modes of motor 2. For example, motor2 may be operated under conditions for which a safety mode is preferred;and motor current should be detected and limited. When operating in thesafety mode, controller 5 enables current detector 4 to sense the motorcurrent. However, during other modes, such as during diagnostic testing,assembly, and the like, it may be desired to disable current detector4--and controller 5 thus does not supply the aforementioned enablesignal to the current detector. As a result, high motor currents willnot be sensed and the current detector will not function to initiate areduction in motor current, a reversal in motor current or aninterruption of motor current.

One embodiment of motor control apparatus to implement the block diagramshown in FIG. 1 is illustrated in FIG. 2. Although particular circuitsand subcircuits are illustrated as being included in specific functionalblocks, it will be appreciated that such designations merely are forconvenience and are not intended to limit the present invention solelythereto. For example, control logic circuit 6 is illustrated as beingincluded in motor drive circuit 3. It will be appreciated that it isequally appropriate to consider the control logic circuit as included incontroller 5. In any event, for convenience, the circuits andsubcircuits described below are assumed to be included in the circuitblocks shown in FIG. 2.

Motor drive circuit 3 is shown as being comprised of control logiccircuit 6, a bias circuit 7 and a power control circuit 8 of variableimpedance switches 9-12 which, in the present embodiment, areillustrated as an H-configuration of field effect transistors (FET's).The control logic circuit, bias circuit and power control circuit allare coupled to a voltage supply of operating potential V_(cc) to beenergized thereby. This source of operating potential also supplies thedrive current I_(M) for motor 2.

Control logic circuit 6 is adapted to receive control signals fromcontroller 5 and functions to decode those control signals for selectingparticular ones of switches 9-12 to be energized. It will be recognizedthat, although FET's are shown and described herein, other suitabledrive elements may be used, such as bipolar transistors or othervariable impedance switching elements.

Let it be assumed that the control signals supplied by controller 5 tocontrol logic circuit 6 are 2-bit control signals. The followingschedule summarizes the switch pairs that are energized and theresulting motor operation:

    ______________________________________                                        Control Signal                                                                          Energized Switches                                                                            Motor Operation                                     ______________________________________                                        01         9, 12          Forward motion                                      10        10, 11          Reverse motion                                      11        9, 10 (or 11, 12)                                                                             Braking                                             00        None            Stop                                                ______________________________________                                    

Bias circuit 7 is coupled to control logic circuit 6 and suppliesenergizing (or drive) signals to those switches 9-12 which have beenselected by controller 5. The bias circuit also is adapted to respond toan overload output signal produced by current detector 4 to vary thebias voltages applied thereby to switches 9-12 and thus vary theimpedances or conductivity of such switches. As the bias circuitdecreases the conductivity of the switches, the magnitude of the motorcurrent supplied therethrough also decreases. Thus, by controlling biascircuit 7, the magnitude of the motor current may be controlled.

Assuming that switches 9-12 are FET's, these FET's are connected asfollows: the gate of each FET is coupled to bias circuit 7 to receive anenergizing signal produced by control logic circuit 6 and a biasvoltage. The drain electrodes of FET's 9 and 10 are connected in commonto voltage supply +V_(cc), and the source electrodes of thesetransistors are connected to the drain electrodes of FET's 11 and 12.The source electrodes of FET's 11 and 12 are connected in common tocurrent detector 4. Motor 2 is connected across the junction defined byFET's 9, 11 and 10, 12.

FET's 9-12 are energized in mutually exclusive pairs. It will berecognized that when FET's 9 and 12 are energized by the control signaldecoded by control logic circuit 6, current flows from voltage supply+V_(cc) to FET 9, through motor 2 and through FET 12 to current detector4. Alternatively, when FET's 10 and 11 are energized, current flows fromthe voltage supply through FET 10, through motor 2 and through FET 11 tocurrent detector 4. The motor current thus is reversed in polarity so asto reverse the operation of the motor. If FET's 9 and 10 are energized,a short circuit is connected across motor 2 to dynamically brake thatmotor. A similar dynamic braking effect is achieved if FET's 11 and 12are energized. Of course, when none of the FET's is energized, nocurrent flows from the voltage supply to the motor. As mentioned above,the magnitude of the current which flows through motor 2 is a functionof the conductivity of the energized FET's which, in turn, is determinedby bias circuit 7.

Current detector 4 is comprised of a comparator 13 which functions tocompare a voltage derived from motor current I_(M) to a referencevoltage V_(ref) which represents a maximum permitted motor current. Inthis regard, a resistor 13B is connected in series with power controlcircuit 8 such that the current which flows through the motor and thepower control circuit also flows through resistor 13B to a referencepotential, such as ground. The voltage V_(R) developed across resistor13B represents the level of the motor current and is coupled to oneterminal, such as the non-inverting input, of comparator 13. Referencevoltage V_(ref) is coupled to the other terminal, shown as the invertinginput, of the comparator. As one example, comparator 13 may be adifferential amplifier, an operational amplifier, or the like. Theoutput of comparator 13 is coupled by a resistor 14 to a transistor 15,shown, for example, as an npn transistor, whose collector electrode isconnected to bias circuit 7. In the illustrated embodiment, when thevoltage V_(R) indicative of motor current I_(M) , exceeds the permittedcurrent level represented by reference voltage V_(ref), a positiveoutput is produced by comparator 13 which renders transistor 15conductive to supply a low signal level to bias circuit 7.

Comparator 13 is illustrated as having an energizing input coupled tovoltage supply V_(cc) by way of a switch 13A which, when closed,energizes the comparator and thus enables it to operate. Switch 13A maycomprise any conventional switching device, such as a transistor switch,or the like, and is adapted to be closed by a mode signal supplied fromcontroller 5. This mode signal may be thought of as an enable signal toenable current detector 4 to operate and is produced when the circuitryoperates in the safety mode.

Controller 5 is adapted to supply a suitable control signal to controllogic circuit 6 in response to a selected mode of operation for motor 2.A predetermined control signal is supplied in response to a detectedoverload condition. As shown, control signal generator 19 receives themode signal which, for the safety mode, is a logic low signal thatserves to close switch 13A and, thus, enable comparator 13. This safetymode signal also enables control signal generator 19 to generate thecontrol signal which reverses or stops motor 2, as may be desired. Thus,when the mode signal level is low, motor overload is prevented.

Controller 5 also includes a timer 16 having an inverting enabling inputcoupled to transistor 15 and a timing input coupled to receive timingpulses generated by a suitable clock circuit (not shown). It isrecognized that timer 16 is enabled by transistor 15 to count thesetiming pulses. When a predetermined pulse count is reached, that is,after a predetermined time duration following the enabling thereof,timer 16 produces an output signal. The timer also includes a clearinput CLR which, when supplied with a logic high signal, as when themode signal is high, inhibits timer 16 from responding to the output oftransistor 15. The output of timer 16 is coupled to one input of an ANDgate 17, the other input of which is an inverting input coupled totransistor 15. The output of AND gate 17 is coupled to one input of 18whose other input is an inverting input connected to receive the modesignal. It is appreciated that AND gate 18 is enabled when the modesignal is low, as when motor 2 operates in the safety mode, but this ANDgate is disabled when the mode signal is high. The output of AND gate 18is coupled to control signal generator 19 and is adapted to trigger thecontrol signal generator to generate the control signal by which motor 2is reversed or, if desired, stopped.

The manner in which the motor control apparatus illustrated in FIG. 2operates now will be described. Let it be assumed that the motoroperates in its safety mode. Thus, the mode signal is a logic lowsignal. Although not shown herein, it will be recognized that anysuitable external circuitry, such as a microprocessor or host machine,may be used to supply the mode signal. In any event, when the modesignal is low, switch 13A is closed and timer 16 and AND gate 18 areenabled.

Let it be further assumed that the control signals being generated bycontrol signal generator 19 energize FET's 10 and 11, resulting in motorcurrent I_(M) flowing through motor 2 in the illustrated direction. Thismotor current also flows through resistor 13B of current detector 4 toproduce a current level voltage V_(R) representative of the magnitude ofthis motor current. If the current level voltage V_(R) is less than thereference voltage V_(ref), comparator 13 produces a low output signal torender transistor 15 nonconductive. It is desirable to select thereference voltage V_(ref) as the maximum, safe motor current level. If,as an example, motor 2 is obstructed, the motor draws more current fromvoltage source +V_(cc) and the motor current I_(M) increases. If thismotor current I_(M) increases so as to overload the motor such thatV_(R) >V_(ref), comparator 13 produces a logic high signal output.Consequently, transistor 15 is rendered conductive to supply a logic lowsignal to bias circuit 7 and to timer 16. The bias circuit responds tothis logic low signal to increase the impedance of switches 9-12, suchas by reducing their conductivity. Consequently, the motor current I_(M)decreases and this alleviates the overload condition.

If the motor current I_(M) is reduced sufficiently, V_(R) <V_(ref) andtransistor 15 is turned off. However, if the reduction in motor currentdoes not result in V_(R) <V_(ref), transistor 15 remains conductive.Timer 16 is enabled when transistor 15 is turned on and timing pulsesare counted. After the timer counts a predetermined number of suchpulses, an output signal is supplied to AND gate 17. The AND gate hadbeen enabled by the logic low signal produced by transistor 15; and ifthis logic low signal remains, the output of timer 16 passes through ANDgate 17 and through previously enabled AND gate 18 to trigger controlsignal generator 19. The control signals now produced by the controlsignal generator may be such that all of switches 9-12 are deenergizedand motor 2 coasts to a stop. Alternatively, these control signals maybe such as to de-energize switches 10 and 11 and energize switches 9 and12 so as to reverse the operation of the motor. It is expected that suchreversal will remove the obstruction or other condition which resultedin motor overload.

If motor current I_(M) is reduced below the detected overload conditionprior to the time that timer 16 counts the predetermined number ofpulses, transistor 15 will be rendered non-conductive to disable thetimer and AND gate 17. Thus, a trigger signal will not pass from thetimer through AND gates 17 and 18 to control signal generator 19. Hence,there will be no change in the current supplied to the motor, other thanthe reduction in motor current level by bias circuit 7, as discussedpreviously.

It is appreciated that timer 16 is adapted to prevent control signalgenerator 19 from being triggered erroneously when motor current I_(M)undergoes relatively brief high level excursions. For example, the motorcurrent will exhibit a high level for a relatively brief time durationwhen motor 2 first is turned on. It is preferred that the operation ofthe motor not be reversed or stopped immediately following this turn-on.Timer 16 prevents this. Timer 16 also avoids undesired changeover inmotor operation that might otherwise occur in response to spuriouscurrent spikes that may be present in the motor current.

While timer 16 has been described as a pulse counter, it is appreciatedthat alternative timing devices may be provided, such as a conventionaldelay line or other timing circuit adapted to time out after apredetermined duration initiated by the output signal produced bycomparator 13.

For non-safety operating modes of motor 2, the mode signal is a logichigh signal to open switch 13A, thereby disabling comparator 13. Thislogic high mode signal also resets and maintains timer 16 in its resetcondition. Hence, the timer does not supply a time-out signal to ANDgate 17. Furthermore, the logic high mode signal disables AND gate 18,thereby preventing a spurious trigger signal from being applied tocontrol signal generator 19. Control signal generator 19 merely respondsto the logic high mode signal to supply control signals to control logiccircuit 6 which, in turn, applies energizing signals to a selected pairof switches by way of bias circuit 7. Moreover, by disabling theoperation of comparator 13, no output signal is produced thereby andtransistor 15 remains non-conductive. Hence, bias circuit 7 does notincrease the impedance (or reduce the conductivity) of switches 9-12.Consequently, in non-safety modes of operation, motor current need notbe limited by the bias circuit under the control of current detector 4.

Another embodiment of current detector 4 and motor drive circuit 3 isillustrated in FIG. 3. Controller 5 is omitted from the illustratedembodiment; and current detector 4 and control logic circuit 6 are seento be substantially the same in FIG. 3 as in FIG. 2.

The switches included in power control circuit 8 are comprised ofbipolar transistors 9-12 in place of the FET's discussed above inconjunction with FIG. 2. Nevertheless, as was the case for the FIG. 2embodiment, transistors 9-12 are energized in respective pairs tocontrol the direction of current flow through motor 2. In addition, andas was discussed previously, motor 2 is dynamically braked if transistorpair 9, 10 is energized or if transistor pair 11, 12 is energized.Similarly, when all of the transistors are de-energized, motor currentis interrupted and motor 2 coasts to a stop.

Bias circuit 7 is illustrated in greater detail and is comprised ofswitches 7A-7D connected to the base electrodes of transistors 9-12,respectively, and diodes 7E-7H having their anodes connected to the baseelectrodes of transistors 9-12, respectively, and their cathodesconnected in common to the collector of transistor 15. Switches 7A-7Dare illustrated as mechanical switches, but it will be recognized thateach switch preferably is a conventional solid-state switching device.Control logic circuit 6 functions to decode the control (or command)signals supplied thereto by controller 5 (not shown); and the decodedsignals serve to energize, or close, respective ones of switches 7A-7D.These switches are connected in common to a source of operatingpotential 1A which supplies a voltage V₈ through each closed switch tothe base electrode of the transistor connected thereto. For example, todrive motor 2 in the forward direction, switches 7A and 7C are closedsuch that energizing potential is supplied from source 1A through switch7A to the base electrode of transistor 10 and through switch 7C to thebase electrode of transistor 11. Transistors 10 and 11 thus areenergized to supply current from source 1A through transistor 10 tomotor 2 and from motor 2 through transistor 11 to resistor 13B ofcurrent detector 4. If switches 7A and 7C are opened and switches 7B and7D are closed, transistors 10 and 11 are deenergized while transistors 9and 12 are turned on to supply current to motor 2 in the reversedirection. Control logic circuit 6 responds to the command signalssupplied thereto to close those switches 7A-7D determined by the commandsignals and thereby direct the motor current I_(M) flowing through motor2.

In operation, if the motor current I_(M) flowing through motor 2 andresistor 13B exceeds a predetermined level, thus indicative of anoverload condition, the current level signal V_(R) produced acrossresistor 13B exceeds the reference voltage V_(ref), whereupon comparator13 produces a logic high output signal. This signal renders transistor15 conductive to supply a logic low signal to the common-connectedcathodes of diodes 7E-7H. Consequently, these diodes are forward biassedto shunt a substantial portion of the energizing voltage from the baseelectrodes of those transistors which are conducting, and also to applyrelatively low base voltages to the non-conducting transistors. As aresult, the base bias voltages of the transistors in power controlcircuit 8 are reduced to reduce their conductivity and increase theimpedance exhibited by each. Therefore, the magnitude of the currentflowing through those transistors which are energized by control logiccircuit 6, such as the current flowing through transistors 10 and 11, isreduced. This, in turn, reduces the motor current I_(M) and alleviatesthe overload condition. Hence, possible damage to the motor and otherapparatus is avoided.

Consistent with the foregoing example, let it be assumed that theimpedance of resistors 10 and 11 are represented as R₁₀ and R₁₁, theresistance of motor 2 is represented as R_(M) and the resistance ofresistor 13B is represented as R. It is seen that such resistances R₁₀,R₁₁ and R are connected in series across voltage source 1A. Whencomparator 13 produces an output signal to render transistor 15conductive, diodes 7F and 7G conduct to reduce the base voltages oftransistors 10 and 11, thereby increasing the effective resistances R₁₀and R₁₁. Hence, the maximum current I_(max) that may flow from voltagesource 1A through motor 2 may be expressed as follows: ##EQU1##

Thus, bias circuit 7, and particularly diodes 7E-7H, increase theresistance presented by H-configuration transistors 9-10 to limit thecurrent I_(M) flowing through motor 2 when an overload condition isdetected by comparator 13. The motor current thus is effectivelycontrolled to prevent damage.

FIGS. 4A and 4B illustrate one embodiment of apparatus in which themotor control circuit of the present invention finds ready application.In particular, FIGS. 4A and 4B depict a video camera 20 which may be aportable video camera, such as an 8mm video camera, having a VTRincorporated therein. Video camera 20 includes a cassette holder 24adapted to receive a video tape cassette 27 and movable between a pop-upload/unload position, as shown in FIG. 4A, and a retracted position,shown in FIG. 4B, whereby the tape cassette 27 may be loaded ontocassette holder 24 and then located at a record/playback position toenable video signals to be recorded on and played back from the tape bythe VTR (not shown).

Video camera 20 includes a housing 21, a camera lens 22 and a cassettecompartment door 23 having a window 23a. Cassette holder 24 supportssupply and take-up reels 26a and 26b and is formed with an end plate 25to shield tape cassette 27 when the cassette is loaded into the cassettereceiving compartment of the camera. The motor described above inconjunction with FIGS. 2 and 3 is used to drive cassette holder 24 fromits load/unload position shown in FIG. 4A to its retracted positionshown in FIG. 4B. This is carried out as a safety mode operation toprevent injury to the user of camera 20 and to prevent the motor whichdrives holder 24 from being overloaded and from burning out.

Typically, tape cassette 27 is loaded onto cassette holder 24,whereafter door 23 is closed and the motor (not shown) drives thecassette holder to its retracted position. A switch may be actuated whendoor 23 is closed to change over the mode signal shown in FIG. 2 to alogic low signal, thereby enabling comparator 13 and timer 16, asdescribed above.

If, while cassette holder 24 is driven to its retracted position, theoperator inadvertently places his finger in the path of travel of thecassette holder, such as shown in FIG. 4B, plate 25 will pinch theuser's finger. This obstruction will prevent holder 24 from reaching itsretracted position and the current supplied to motor 2 will increase toits overload level. If the user does not take corrective action for apredetermined time, such as on the order of 1-2 seconds; for example, ifhe does not remove his finger or if he does not attempt to eject thetape cassette, timer 16 times out to trigger control signal generator19. The control signal generator then supplies command signals tocontrol logic circuit 6 which, in one embodiment, deenergizes the FET's,transistors or other variable impedance switches included in powercontrol circuit 8, whereupon motor 2 stops. In an alternativeembodiment, the command signals supplied to the control logic circuitfrom control signal generator 19 reverse the pair of switches which hadbeen energized theretofore, thereby reversing the direction of currentflow through motor 2 and reversing the motor. In this alternativeembodiment, cassette holder 24 now is driven automatically to return toits load/unload position.

In addition to the foregoing command signals produced by control signalgenerator 19 in response to the output signals produced by comparator 13and timer 16, the comparator output also is used to increase theimpedance of the variable impedance switches so as to reduce the levelof motor current I_(M). This immediate reduction in motor currentremoves the overload condition and reduces the force that may be exertedon the user's finger by the motor-driven cassette holder.

While the present invention has been particularly shown and describedwith reference to preferred embodiments, it will be readily apparent tothose of ordinary skill in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe invention. For example, controller 5 may be implemented by a simplemicroprocessor, such as the microprocessor that normally is included inthe equipment with which the present invention is used, such as in videocamera 20. Also, although an H-configuration of variable impedanceswitches has been discussed for supplying drive current to motor 2, itis recognized that other configurations may be used, such as a simplepower amplifier connected in series with the motor. Furthermore, thepresent invention is not limited solely to the FET's or bipolartransistors which have been discussed above. Alternative switchestypically used to supply drive current to a motor may be used.

It is, therefore, intended that the appended claims be interpreted asincluding the embodiments described herein, the alternatives mentionedabove and all equivalents thereto.

What is claimed is:
 1. Video cassette loading apparatus, comprising: acassette holder movable between a cassette receiving position and aretracted position; controllable loading means for driving said cassetteholder between said positions; detecting means for detecting abnormaloperation of said loading means caused by an overload; and protectionmeans coupled to said loading means for controlling the operation ofsaid loading means to alleviate said overload by reducing a drivingforce exerted on said cassette holder without removing said drivingforce and, if the reduction in the driving force on said cassette holderfails to alleviate said overload, to selectively stop driving of saidcassette holder or reverse the direction of said driving of the cassetteholder.
 2. The apparatus of claim 1 wherein said loading means comprisesan electric motor; said detecting means comprises current sensing meansfor sensing motor current whose level is indicative of said overload;and said protection means comprises current control means forcontrolling current in said motor.
 3. The apparatus of claim 2 whereinsaid loading means further comprises a power source, and variableimpedance switch means selectively energized for coupling said motor tosaid power source; and said current control means comprises bias meansfor varying the impedance of said switch means to limit current suppliedtherethrough to said motor.
 4. The apparatus of claim 3 wherein saidvariable impedance switch means comprises plural transistor meansconnected in H-configuration to couple said motor to said power sourceand selectively energized to supply current to said motor in first orsecond directions.
 5. Video cassette loading apparatus, comprising: acassette holder movable between a cassette receiving position and aretracted position; controllable loading means for driving said cassetteholder between said positions and which includes an electric motor, apower source and variable impedance switch means selectively energizedfor coupling said motor to said power source; detecting means fordetecting abnormal operation of said loading means caused by an overloadand which comprises current sensing means for sensing motor currentwhose level is indicative of said overlaod including current levelgenerating means for generating a current level signal representative ofmotor current, reference means for providing a reference signalrepresentative of a reference motor current, and comparator means forcomparing said current level signal to said reference signal to producean output signal if said current level signal exceeds said referencesignal; and protection means coupled to said loading means forcontrolling the operating of said loading means to alleviate saidoverload by reducing a driving force exerted on said cassette holderwithout removing said driving force and, if the reduction in the drivingforce on said cassette holder fails to alleviate said overload, toselectively stop driving of said cassette holder or reverse thedirection of said driving of the cassette holder, said protection meansincluding current control means having bias means for varying theimpedance of said switch means to limit current supplied therethrough tosaid motor, said output signal from said comparator means being suppliedto said bias means to vary the impedance of said switch means.
 6. Theapparatus of claim 5 wherein said current control means furthercomprises timer means for producing a time-out signal if said comparatormeans produces said output signal for a predetermined time duration, andcontrol signal generating means coupled to said timer means forgenerating control signals in response to said time-out signal fordetermining the current supplied through said switch means to saidmotor.